It is known to remove or recover nitrogen oxides (NO.sub.x) from flue gases using an adsorbent such as active carbon or active coke in conjunction with a reducing agent in the form of ammonia, carbon monoxide or hydrogen, or mixtures thereof, to limit the nitrogen oxide levels in gases which are vented to the atmosphere.
The flue gases in which these processes are practiced are generally flue gases which arise from the combustion of low sulfur fuels, or combustion gases in which the sulfur dioxide content has been reduced in a prior desulfurization apparatus utilizing wet, semidry or dry desulfurization processes.
Adsorbents which can be used to remove the sulfur dioxide component can be solutions or suspensions of calcium, magnesium, sodium or ammonia-based compounds as is described, for example, in W. Kaminsky, Chem. Ing.-Technik 55 (1983), pages 673 ff.
At the present time, apart from the elimination of sulfur dioxide from flue gases which are vented into the atmosphere, it is imperative to reduce or eliminate NO.sub.x emissions.
In German patent document DE-OS. No. 29 11 712, for example, a process is described in which the flue gases pass through a traveling bed of carbon-containing adsorbent with metered amounts of gaseous ammonia. A reducing agent is added to catalytically reduce the nitrogen oxides to nitrogen with the production of water vapor.
This process has the disadvantage that the introduction of the reducing agent into the flue gas before the resulting mixture enters the bed can effect a sufficient removal of nitrogen oxides, i.e. a nitrogen oxide removal efficiency above 60%, only when the flue gas is provided with a significant excess of the reducing agent above the stoichiometric amount required for the theoretical reduction of all of the nitrogen oxide content of the flue gas.
It is an obvious disadvantage that excess reducing agent must be used to obtain the desired degree of nitrogen oxide removal.